1. Technical Field of the Invention
The present invention relates to lasers used for material processing, and more particularly to the design and use of a spacial filter aperture external to the laser to achieve increased speed in laser machining operations.
2. Prior Art
A major use for high power lasers is material processing. In particular, high power lasers are used for cutting and welding of metals. The initial penetration of a focused laser beam into the metal can sometimes take a significant amount of time. This is especially true when a continuous high power CO2 laser is used to cut or weld reflective metals such as aluminum. This initial penetration delay also results in an enlarged hole and a large heat affected zone. In some cases, the problem can be so extreme that it is not possible to make the initial penetration in the middle of a piece of metal. Only if the penetration is initiated at the edge of the metal, can the laser be used. This problem has merely been accepted as an unavoidable result of the high reflectively of the metal. In the case of aluminum, coatings have been developed which can decrease the surface reflectivity and thereby assist in this penetration phase. However, these coatings present an extra step in application and removal. Also in the case of laser welding, coatings can introduce contamination into the weld area.
The invention described herein involves the use of an external spacial filter. In the prior art, a spacial filter is sometimes used with low power lasers such as a helium neon laser to eliminate imperfections in the beam. For example, in holography, it is desirable to utilize a homogenous laser beam. In this case, the laser beam can be passed through a spacial filter which loses some power while eliminating intensity variations in the laser beam. This is an entirely different objective and result than the effect described herein. The spacial filter usually consists of: 1) a focusing lens which brings the laser beam to a focus, 2) an aperture with a very small hole and 3) a collimating lens. The small hole in the aperture is positioned at this focus so that only light passing through this small hole continues on. The laser beam then expands and passes through the collimating lens. The power of these lasers is usually in the milliwatt range and sometimes can reach a few watts. The use of these low power lasers with spacial filters is for applications which requires a homogeneous beam. The term "spacial filter" implies that this is a filter which eliminates unwanted light distributed in space.
In the prior art applications, a spacial filter has been used as a passive element which does not influence the laser performance. In this invention, the external spacial filter becomes part of the resonant system and changes the laser characteristics as the laser is performing useful material processing work. In materials processing with high power continuous lasers, there has never been any reason to utilize spacial filters. They introduce extra expense and a loss of power without any obvious benefit. The present invention utilizes a spacial filter to obtain a new effect which benefits material processing. Since material processing requires high power, typically 500 to 5,000 watts, the prior art spacial filters used with low power lasers are completely unsuitable for use with the high power densities used with materials processing lasers.